The research presents a study of explanations of physics teachers in college classes. The context is the Faculty of Engineering at the University of Carabobo, Venezuela. The methodological approach is qualitative. This is a descriptive and interpretive cases. Given the social, rhetorical and argumentative science classes, we selected as the theoretical reference provided by Perelman and Olbrechts-Tyteca (1989). The analytical and methodological proposal derived from the same to identify the resources used by teachers as «reasons» for students credible scientific knowledge presented to them. The study results have implications toward the improvement of teaching practice.